1. Field of the Invention
The present invention relates to a sheet feeding apparatus with duplicate feed preventive function capable of feeding sheets such as recording paper in copying machine or printer one by one.
2. Description of the Prior Art
Typical prior art is shown in FIG. 1 (1) and FIG. 1 (2). In FIG. 1 (1), recording papers 1 in a copying machine are stacked up on a laying plate 2, and the bottom sheet of these sheets is attracted in vacuum to a paper feeding stretch belt 3, and is fed in the paper feed direction 4 to be held by a pair of feed rollers 5, and is further conveyed. The belt 3 has multiple air passage holes, and the vacuum attraction box 6 is in negative pressure, so that the bottom recording paper of the stack of recording papers is attracted in vacuum to the belt 3 as mentioned above. The bottom recording sheet and the second and other sheets are separately vertically by the air flow injected from nozzle holes 8 in air flow forming means 7, so that the recording papers are fed one by one. The recording paper attracted in vacuum and fed by the belt 3 is held by the rollers 5 and conveyed, then the rotation of the belt 3 is stopped, and the sheet is fed only by the function of the rollers 5.
In such prior art, in the initial state shown in FIG. 1 (1), the belt 3 is rotated and driven, and the bottom recording paper is fed in the feed direction 4, and then, as shown in FIG. 1 (2), the bottom sheet indicated by reference number P1 is held by the rollers 5 and is conveyed, when the belt 3 is stopped as mentioned above. While this being 3 is being rotated and driven, if a gap W is produced between the end part of the upstream side of the stopping direction of the bottom recording paper P1 (that is, the rear end part) P1a, and the end part 2a at the downstream side of the stopping direction 4 of the laying plate 2 disposed at the fixed position, the second sheet P2 from the bottom is attracted to contact with the belt 3 at the gap W, and is fed in the feed direction, together with the bottom recording paper P1. The feed effective length W1 working for feed of recording paper by the belt 3 exposed at the downstream side in the feed direction from the end part 2a of the laying plate 2 must be enough to obtain a large conveying force required for the recording paper of large size and large weight, and is hence determined by the distance between the belt and rollers 5 in the feed direction and the minimum length of the recording paper 1.
In the prior art, since such feed effective length W1 is fixed, if the feed effective length W1 is set long in consideration of feeding of recording paper of large size and large weight, when feeding recording paper of small size, the second recording paper P2 is attracted to the portion W not covered by the bottom recording paper P1 in the range of the feed effective length W1 of the belt 3 before the recording paper begins to be fed by the rollers 5, and duplicate feed of recording papers occurs, or the second recording paper is slightly fed in the feed direction 4, and when feeding the next recording paper in this state, the recording papers are not separately securely in the vertical direction by the air injected from the nozzle holes 8, so that duplicate feed occurs, too.
To the contrary, if the feed effective length W1 of the belt 3 is set short in consideration of feeding of recording paper of small size, the conveying force for feeding recording paper of large size and large weight is insufficient, and feeding failure of recording paper of large size and large weight occurs. Hence, in the prior art, the size of the sheets that can be fed is limited.